Interchangeability of Varilrix and Varivax for Second Dose
Yes, Varilrix can be given as the second dose after Varivax, as both vaccines are derived from the same Oka strain of varicella-zoster virus and are considered interchangeable for completing the two-dose varicella vaccination series. 1
Evidence Supporting Interchangeability
Same Viral Strain Foundation
- Both Varilrix (GSK) and Varivax (Merck) are live attenuated vaccines derived from the identical Oka strain of varicella-zoster virus, making them functionally equivalent for immunization purposes 1
- The Advisory Committee on Immunization Practices (ACIP) recommends two 0.5-mL doses of varicella vaccine administered subcutaneously for children aged >12 months, without specifying that the same brand must be used for both doses 2
Dosing Interval Requirements
- For children aged 12 months to 12 years, the minimum interval between doses is 3 months 2
- If the second dose is inadvertently administered between 28 days and 3 months after the first dose, it is considered valid and does not need to be repeated 2
- For persons aged ≥13 years, the minimum interval is 4 weeks 2
Immunogenicity and Efficacy Considerations
Two-Dose Regimen Benefits
- A two-dose varicella vaccine regimen achieves >99% seroconversion rates (antibody response ≥5 gpELISA units) compared to 76-85% after a single dose 2
- Vaccine efficacy after two doses is 98.3% compared to 94.4% after a single dose, representing a 3.3-fold lower risk of breakthrough disease 2
- The second dose provides a greater-than-tenfold boost in geometric mean antibody titers when measured 6 weeks after administration 2
Comparative Performance Data
- Recent modeling studies comparing V-MSD (Varivax) and V-GSK (Varilrix) estimated that 90.3% of recipients gained permanent protection after the first dose of Varivax compared to 61.7% with Varilrix 3
- However, after two doses, protection was similar: 97.0% for Varivax and 93.8% for Varilrix, demonstrating that the second dose effectively equalizes protection regardless of which vaccine is used first 3
Safety Profile
Second Dose Tolerability
- The two-dose vaccine regimen is generally well-tolerated with a safety profile comparable to the one-dose regimen 2, 4
- Injection site reactions are slightly higher after the second dose (25.4%) compared to the first dose (21.7%), but systemic reactions are actually lower 2
- Fever incidence decreases from 7% after dose 1 to 4% after dose 2, and varicelliform rash decreases from 3% to 1% 2
Practical Implementation
No Need for Brand Consistency
- There is no requirement in ACIP guidelines to use the same vaccine brand for both doses 2
- The focus is on completing the two-dose series with appropriate intervals, not on brand consistency 2
Common Pitfall to Avoid
- Do not delay or repeat vaccination unnecessarily due to concerns about mixing vaccine brands—both are Oka-strain vaccines and are functionally interchangeable 1
- Do not confuse this situation with other vaccines where brand consistency may be more critical; varicella vaccines are uniquely similar due to their shared viral origin 1